make Perl hash iteration reproducible

By default Perl doesn't iterate over hashes in a reproducible manner,
to avoid DDoS. Since we aren't providing untrusted inputs, it is fine to
disable this behaviour.

Among other things, this makes the .dbd and some .h generation reproducible.

More information here:

- https://reproducible-builds.org/docs/stable-outputs/
- https://perldoc.perl.org/perlrun#PERL_HASH_SEED

configure/CONFIG

@@ -34,6 +34,11 @@ ifeq ($(origin EPICS_HOST_ARCH), undefined)
   EHA :=
 endif
 
+# Make Perl hash iteration reproducible.
+# See: https://reproducible-builds.org/docs/stable-outputs/
+#
+export PERL_HASH_SEED = 0
+
 -include $(CONFIG)/RELEASE
 -include $(CONFIG)/RELEASE.$(EPICS_HOST_ARCH)
 -include $(CONFIG)/RELEASE.$(EPICS_HOST_ARCH).Common